Renewable energy generation from forest residues is commonly practiced in the forest products industries. The U.S. forest products industry consumed 27.1 million tons of wood derived biomassin the generation of steam. By comparison, the power generation industry used 11.9 million tons of biomass of which 80% is wood derived. The biomass consumption in energy and power generation is expected to double in every 10 years until 2030.
The major components of cellulosic biomass are lignin, hemicelluloses and cellulose. The forest products industry practices the addition of steam to wood chips, to dissolve predominantly hemicelluloses at temperatures above 160 degrees C.; this process is termed “steam explosion”. Hemicelluloses removed in this process is termed “extract”. A concentration of the extract through evaporation is energy intensive, although it is currently practiced in industry to produce molasses.
Previous research indicates that ethanol, acetic acid and their byproducts can be derived from the extract. Hardwood in particular, and softwood to a lesser extent produces an extract rich in acetic acid and sugars as taught by Amidon et al. in (U.S. Patent Application No. 2007/0079944 A1, Apr. 12, 2007).
The present inventors found, inter alia, an alternative method to extract hemicelluloses from biomass prior to thermal conversion of the biomass to energy and have developed a process wherein the hemicelluloses in the extract can be converted to alcohol and other chemical bioproducts in an energy efficient process.
In a further embodiment, there is disclosed enzymatic conversion of cellulosic fiber to glucose and other monomeric sugars and specifically the re-utilization of existing process equipment in pulp and paper mills.
The current practice in proposed cellulosic ethanol processes is to add enzymes to 6-15% solids cellulosic fiber stock, termed medium consistency stock, and wait for completion of cellulose hydrolysis in 24-72 hours. This process is inefficient, because mixing of medium consistency stock consumes disproportionally more energy than mixing of low consistency (1-6% solids) stock. However, the equipment required for the storage and processing of low consistency stock are larger than for medium or high consistency (16-35% solids) stock.
The activity of the enzymes reduces upon time, because of binding to non-specific sites, e.g., lignin. Mixing at high consistency is not efficient and slows enzymatic reaction. Over the hydrolysis period, the stock consistency decreases thus improving both mixing efficiency and enzyme activity, however high dissolved sugar concentration from the hydrolyzed cellulose fiber has a negative impact on enzyme activity.
Phillips et al. have taught in U.S. patent application “HIGH CONSISTENCY ENZYMATIC HYDROLYSIS FOR THE PRODUCTION OF ETHANOL” (US20120036768) a method of converting biomass to sugars using a two-step process, where enzymes are adsorbed on biomass at approximately 5% consistency for 5-10 minutes and then dewatered to 20-30% consistency for 24-48 hours. Phillips et al. further taught that these steps can be repeated and the filtrate recycled to the previous step.
The current inventors have discovered, inter alia, that a stepwise addition of enzymes improved the hydrolysis yields at lower enzyme dosage and at low consistency typical of the stock consistencies used in pulp and paper mills. This advantageous because existing pulp and paper mill equipment and vessel infrastructure can be redeployed.